Over the last 20 years there has been substantial interest in the use and manipulation of single photon sources for quantum information processing.
Over the last decade, optically-active defects in diamond have been demonstrated as efficient and robust solid-state single photons sources owing to long decoherence times and strong optical dipole coupling at room temperature. Many optically active defects have been studied in diamond including silicon-vacancy (SiV), nickel and chromium-related defects, and the negatively charged nitrogen-vacancy (NV−) defect. The NV− defect in particular has attracted a lot of interest because its spin state can be coherently manipulated with high fidelity owing to an extremely long coherence time (which may be quantified and compared using the transverse relaxation time T2), because of its specific electronic structure, and because of the availability of optical pumping. Moreover, the electron spin states of NV− defects can be read out through photons, which is a key ingredient towards using the NV− defects as qubits for long-distance quantum communications and scalable quantum computation. Such results make the NV− defect a competitive candidate for solid-state quantum information processing (QIP).
One major problem in producing materials suitable for quantum applications is preventing the qubits from decohering, or at least lengthening the time a system takes to decohere (i.e. lengthening the “decoherence time”). A long T2 time is desirable in applications such as quantum computing as it allows more time for the operation of an array of quantum gates and thus allows more complex quantum computations to be performed.
WO 2010010344 discloses that single crystal diamond material which has a high chemical purity, i.e. a low nitrogen content, and wherein a surface of the diamond material has been processed to minimise the presence of crystal defects, can be used to form a solid state system comprising a quantum spin defect. Where such materials are used as a host for quantum spin defects, long T2 times are obtained at room temperature and the frequency of the optical transitions used to read/write to devices are stable.
WO 2010010352 discloses that by carefully controlling the conditions under which diamond material is prepared using chemical vapour deposition (CVD) methods, it is possible to provide diamond material which combines a very high chemical purity with a very high isotopic purity. By controlling both the chemical purity and the isotopic purity of the materials used in the CVD process, it is possible to obtain synthetic diamond material which is particularly suitable for use as a host for a quantum spin defect. Where such materials are used as a host for quantum spin defects, long T2 times are obtained at room temperature and the frequency of the optical transitions used to read/write to the devices are stable. A layer of synthetic diamond material is disclosed which has a low nitrogen concentration and a low concentration of 13C. The layer of synthetic diamond material has very low impurity levels and very low associated point defect levels. In addition, the layer of synthetic diamond material has a low dislocation density, low strain, and vacancy and self-interstitial concentrations which are sufficiently close to thermodynamic values associated with the growth temperature that its optical absorption is essentially that of a perfect diamond lattice.
In light of the above, it is evident that WO 2010010344 and WO 2010010352 disclose methods of manufacturing high quality “quantum grade” single crystal diamond material. The term “quantum grade” diamond is used herein for diamond material which is suitable for use in applications that utilize the material's quantum spin properties. Specifically, the quantum grade diamond material's high purity makes it possible to isolate single defect centres using optical techniques known to the person skilled in the art.
One problem is that the single photon emission from defects in such materials can be very weak. For example, NV− defects in diamond exhibit a broad spectral emission associated with a Debye-Waller factor of the order of 0.05, even at low temperature. Emission of single photons in the Zero-Phonon Line (ZPL) is then extremely weak, typically of the order of a few thousands of photons per second. Such counting rates might be insufficient for the realization of advanced QIP protocols based on coupling between spin states and optical transitions within reasonable data acquisition times. This problem is also evident for other photon emitting defects within diamond.
In addition to the problem of weak emission, it is evident that the high refractive index of diamond means that due to total internal reflection very few photons can be collected within a small solid angle. Accordingly, there is a need to increase the light collection from single photon emitters in diamond for applications that include magnetometry and quantum information processing. In this regard, both WO 2010010344 and WO 2010010352 disclose that the quantum grade diamond material discussed therein may have a surface which has a macroscopic curvature, e.g. a lens with a radius of curvature of between about 10 μm and about 100 μm to collect and focus the light output from a quantum defect centre. It is suggested that an isotropic etch may be used to form macroscopic curved features, such as a lens, whilst controlling or reducing the surface roughness.
However, single crystal quantum grade diamond material can be difficult, time consuming, and expensive to manufacture, particular for large single crystals, when compared with lower grades of diamond. As such, it is difficult, time consuming, and costly to form a single crystal quantum grade diamond of sufficient size to incorporate an optical out-coupling structure such as a lens therein.
It is an aim of certain embodiments of the present invention to solve the aforementioned problem.